Effects of thermal annealing on the optoelectronic properties of hydrogenated amorphous germanium

In this work we investigate the effect of thermal annealing above the deposition temperature (200 C) on the optoelectronic properties of hydrogenated amorphous Ge thin films. For this purpose, visible and infrared transmission spectroscopy, photothermal deflection spectroscopy (PDS), Raman scattering, dark conductivity and electron spin resonance (ESR) spectroscopy have been used. For thermal treatments below 350°C, the defect density increases with increasing anneal temperature T_a, but the sample remains amorphous. In this case the absorption coefficient at 0.7 eV determined by PDS can be used as a measure of the dangling-bond spin density (obtained by ESR). For 350°C< T_a<420°C, crystallites are formed and grow, up to T_a= 430°C, when the sample becomes microcrystalline. The crystallization process is best described by a surface-induced crystallization with nucleation occurring at the film substrate interface.